In high-voltage applications, in order to effectively increase the voltage level, IGBTs (Insulated Gate Bipolar Transistor) are connected in series. When the IGBTs are connected in series, voltages among the IGBTs are unbalanced and voltage spike of the IGBTs is higher. An active clamping circuit is usually added between a collector and a gate of the IGBT to improve the reliability of the IGBTs connected in series, and solve the problem that unbalanced voltages are among the IGBTs and voltage spike of the IGBTs is higher.
Accurate control of the gate voltage of the IGBT is an important guarantee for the reliable application of the active clamping circuit.
An active clamping circuit as shown in FIG. 1 is applied to IGBT. The active clamping circuit mainly comprises resistors, capacitors, diodes and more than one Zener diode.
More than one Zener diode, resistors and diodes are connected in series to form a branch 1, which is connected in parallel across the collector and the gate of the IGBT; a resistor and a capacitor are connected in series to form a branch 2, which is connected in parallel across the collector and the gate of the IGBT;
A control method of the active clamping circuit has following features:
the branch 1 responds to the magnitude of a collector-emitter voltage Vce of the IGBT. After the collector-emitter voltage Vce exceeds the reverse breakdown voltage (clamping voltage) of all Zener diodes in the branch 1, the Zener diodes in this branch 1 are broken down, and charges are injected into the gate of IGBT;
the branch 2 responds to the slope of the collector-emitter voltage Vce of the IGBT. When the collector-emitter voltage Vce rises with a certain slope, charges are injected into the gate of the IGBT through the capacitor.
The charges injected into the gate of the IGBT by the branch 1 and the branch 2 can raise a gate voltage Vge above a threshold voltage Vgeth, so that the IGBT enters a turned-on state so as to reduce the collector-emitter voltage Vce of the IGBT.
The advantage of this control method is that it can effectively reduce the voltage spike when the IGBT is turned off. The disadvantages thereof are as follows: the active clamping circuit usually works at the moment when the IGBT is turned off, at this time a lower transistor of a power amplifier circuit in the last stage of a IGBT driving circuit is in an on-state, a large portion of charges injected into the gate of the IGBT are bypassed by the lower transistor, which reduces the effectiveness of the active clamping circuit and adds a significant loss on the Zener diodes. Especially when the active clamping circuit operates frequently, the loss on the Zener diodes is unacceptable.
In addition, there are still many problems and disadvantages in the active clamping circuit in the prior art, and the structure of the active clamping circuit is complex. The problem in the active clamping circuit shown in FIG. 1 has not been solved. Most of charges injected into the gate are bypassed by the lower transistor of the power amplifier circuit, the active clamping circuit is not highly effective. Or when the active clamping circuit works, the IGBT gate voltage Vge is uncontrollable, the gate voltage Vge is easily raised up to a higher magnitude. The IGBT is in on-state and the collector-emitter voltage Vce will drop largely. Then the gate voltage Vge drops and the collector-emitter voltage Vce rises again, such that both the collector-emitter voltage Vce and gate voltage Vge will oscillate.
Therefore, the present disclosure provides a novel control circuit and method for regulating the IGBT gate voltage.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure, and therefore it may contain information that does not form the prior art that is already known to a person skilled in the art.